In a cordless telephone set, etc., a plurality of subsystems are included to provide a radio communication system, wherein each of the subsystems includes a base station and a mobile station. In each subsystem, a pair of channels are used for communication in both directions between the base station and the mobile station. In designating a channel among plural communication channels, a request of communication is transmitted, for instance, from the base station through an exclusive channel to the mobile station. Then, a signal level is detected in a first selected channel at the base station. In this detection, when a predetermined condition is met for communication from the mobile station to the base station, the first selected channel is designated to be used for the communication. In the same manner, a signal level is detected in a second selected channel at the mobile station. In this detection, when a predetermined condition is met for communication from the base station to the mobile station, the second selected channel is designated to be used for the communication. Thus, the first and second selected channels are used for the communication in the both directions between the base and mobile stations.
For the purpose of designating a channel among a plurality of channels, U.S. application Ser. No. 465,320 entitled "a method for connecting channels of decentralized radio system" which has been assigned to the some assignee of this application has been filed on Jan. 16, 1990 by Yukitsuna Furuya.
In this method of connecting channels of decentralized radio systems which method is generally called "a carrier sensing method", each subsystem sets a given channel to monitor a reception signal level. If the reception signal level of the given channel is higher than a threshold value, a next channel is set. When the given channel is an empty channel in the detecting result that the reception signal channel is lower than the threshold value, the communication is started by use of the channel. Each subsystem increases the threshold value, when the channel connection is rejected, while each subsystem decreases the threshold value, when the communication is started. This method will be explained in more detail later.
However, this proposed method has a disadvantage in that a control, under which communication is rejected, occurs even in a state when communication is possible, that is when a threshold value is fixed to be a relatively small value which is usually higher than a noise level by a small predetermined level. On the other hand, when the threshold is set to a high value, a condition of allowing a request of communication is relieved, so that a case in which the communication is interfered by communication in another subsystem is increased with greater frequency.
In order to overcome these disadvantages, a method for connecting a channel between a base station and a mobile station which is generally called "a selection method", in which communication is allowed to start, when a difference between levels of signals supplied from mobile stations in a channel selecting subsystem and a neighboring subsystem is larger than a predetermined level, is being considered for implementation.
Even in this selection method, however, there is a further disadvantage in that communication which is then started in the neighboring subsystem is interfered with by the previously started communication, where a mobile station is far from a base station in the neighboring subsystem, although this will be explained in more detail later.